Method for transmitting and receiving traffic indication map in wireless communication system and device therefor

ABSTRACT

Disclosed are a method for transmitting and receiving a traffic indication map (TIM) in a wireless communication system and a device for supporting the same. More particularly, the method for transmitting a traffic indication map comprises the step of transmitting a TIM to a STA through a beacon frame, wherein the TIM includes: a block bitmap field for indicating a subblock including a STA in which buffered downlink data exists for respective N (N are two or more natural numbers) blocks; and a block control field for indicating an encoding method for the bitmap field.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method of transmitting and receiving a trafficindication map (TIM) in a wireless LAN system and an apparatus therefor.

BACKGROUND ART

Recently, with the help of development of technologies of informationand communication, various wireless communication technologies aredeveloping. Among the various wireless communication technologies, awireless LAN (WLAN) is a technology enabling such a portable terminal asa personal digital assistant (PDA), a laptop computer, a portablemultimedia player (PMP) and the like to access the internet in wirelessin home, enterprise or an area to which a specific service is providedbased on a wireless frequency technology.

In order to overcome a limit for transmission speed, which has beenpointed out as a weak point of the wireless LAN, a latest technologicalstandard introduced a system of which network speed and reliability areincreased and a management distance of a wireless network is enlarged.For instance, IEEE 802.11n supports high throughput where dataprocessing speed is greater than maximum 540 Mbps. Moreover, in order tominimize a transmission error and optimize data speed, a MIMO (multipleinputs and multiple outputs) technology using multiple antennas in botha transmitting end and a receiving end is introduced.

DISCLOSURE OF THE INVENTION Technical Tasks

One object of the present invention is to propose a method oftransmitting and receiving an enhanced TIM structure in a wirelesscommunication system, preferably, in a wireless LAN system and anapparatus therefor.

Another object of the present invention is to propose a TIM structurecapable of indicating an STA (station) in which buffered downlink dataexist in accordance with a plurality of blocks.

The other object of the present invention is to propose a structure forreducing overhead of a TIM bitmap.

Technical tasks obtainable from the present invention are non-limitedthe above-mentioned technical task. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solution

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, accordingto one embodiment, a method of transmitting a traffic indication map(TIM) to a station (STA) in a wireless communication system includes thestep of transmitting the TIM to the STA via a beacon frame, wherein theTIM includes a block bitmap field, which indicates a sub-block includingan STA in which downlink data buffered for each of N (N is a naturalnumber equal to or greater than 2) number of blocks exist, and a blockcontrol field indicating an encoding scheme of the bitmap field.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, according to a different embodiment, amethod of receiving a traffic indication map (TIM), which is received bya station (STA) in a wireless communication system, includes the step ofreceiving the TIM from an access point (AP) via a beacon frame, whereinthe TIM includes a block bitmap field, which indicates a sub-blockincluding an STA in which downlink data buffered for each of N (N is anatural number equal to or greater than 2) number of blocks exist, and ablock control field indicating an encoding scheme of the bitmap field.

Following items can be commonly applied to embodiments according to thepresent invention.

The block bitmap field can have a size of N byte.

The block bitmap field of 1 byte unit can indicate blocks different fromeach other.

The N may correspond to a fixed value in the wireless communicationsystem or a value configured by an access point (AP).

The TIM can further include a block bitmap size field indicating a sizeof the block bitmap field.

The block bitmap size field can indicate the size of the block bitmapfield by N byte.

The block bitmap field and a sub-block bitmap field indicated by theblock bitmap field can be repeatedly appeared by N times in the TIM.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

Advantageous Effects

According to embodiment of the present invention, a method oftransmitting and receiving an enhanced TIM structure in a wirelesscommunication system, preferably, in a wireless LAN system and anapparatus therefor can be provided.

And, according to embodiment of the present invention, a TIM structurecapable of indicating an STA (station) in which buffered downlink dataexist can be provided in accordance with a plurality of blocks.

And, according to embodiment of the present invention, a structurecapable of reducing overhead of a TIM bitmap is provided.

Effects obtainable from the present invention may be non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a diagram for an example of a structure of IEEE 802.11 systemto which the present invention is applicable;

FIG. 2 is a diagram for a different example of a structure of IEEE802.11 system to which the present invention is applicable;

FIG. 3 is a diagram for a further different example of a structure ofIEEE 802.11 system to which the present invention is applicable;

FIG. 4 is a diagram for an example of a structure of WLAN system;

FIG. 5 is a diagram for an example of a data link layer structure and aphysical layer structure of IEEE 802.11 system to which the presentinvention is applicable;

FIG. 6 is a flowchart for explaining a general link setup process in awireless LAN system to which the present invention is applicable;

FIG. 7 is a diagram for an example of a MAC frame format of IEEE 802.11system to which the present invention is applicable;

FIG. 8 is a diagram for an example of an HT format of an HT controlfield in a MAC frame according to FIG. 7;

FIG. 9 is a diagram for an example of a VHT format of an HT controlfield in a MAC frame according to FIG. 7;

FIG. 10 is a diagram for an example of a PPDU frame format of IEEE802.11n system to which the present invention is applicable;

FIG. 11 is a diagram for an example of a VHT PPDU frame format of IEEE802.11ac system to which the present invention is applicable;

FIG. 12 is a diagram for explaining a backoff process in a wireless LANsystem to which the present invention is applicable;

FIG. 13 is a diagram for explaining a hidden node and an exposed node;

FIG. 14 is a diagram for explaining an RTS and a CTS;

FIG. 15 is a flowchart for explaining a power management operation;

FIGS. 16 to 18 are diagrams for explaining an operation of an STA, whichhas received a TIM, in detail;

FIG. 19 is a diagram for an example of a U-APSD coexistence elementformat;

FIG. 20 is a diagram for an example of a TIM element format;

FIG. 21 is a diagram for an example of compression of a TIM elementusing dynamic AID assignment;

FIG. 22 is a diagram for explaining a format of a TIM element;

FIG. 23 is a diagram for explaining a bitmap format of a TIM elementaccording to one embodiment of the present invention;

FIG. 24 is a diagram for explaining a hierarchical structure of a TIMelement;

FIG. 25 is a diagram for an example of an AID structure according to astructure of a hierarchical TIM element;

FIG. 26 and FIG. 27 are diagrams for examples of a format of a TIMelement including a hierarchical structure;

FIG. 28 is a diagram for explaining an example of overhead occurringwhen a subblock bitmap per block is indicated in three contiguousblocks;

FIGS. 29 to 31 are diagrams for explaining examples of a long blockbitmap encoding scheme;

FIG. 32 and FIG. 33 are diagrams for explaining examples of a multipleblock bitmap encoding scheme;

FIG. 34 is a block diagram for a configuration of a wireless deviceaccording to one embodiment of the present invention.

BEST MODE Mode for Invention

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. In the following detailed description of the inventionincludes details to help the full understanding of the presentinvention. Yet, it is apparent to those skilled in the art that thepresent invention can be implemented without these details.

Occasionally, to prevent the present invention from getting vaguer,structures and/or devices known to the public are skipped or can berepresented as block diagrams centering on the core functions of thestructures and/or devices.

Specific terminologies used in the following description are provided tohelp understand the present invention and the use of the specificterminologies can be modified into a different form in a range of notdeviating from the technical idea of the present invention.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of wireless access systems includingIEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE-A (LTE-Advanced)system and 3GPP2 system. In particular, the steps or parts, which arenot explained to clearly reveal the technical idea of the presentinvention, in the embodiments of the present invention may be supportedby the above documents. Moreover, all terminologies disclosed in thisdocument may be supported by the above standard documents.

The following description of embodiments of the present invention may beusable for various wireless access systems including CDMA (code divisionmultiple access), FDMA (frequency division multiple access), TDMA (timedivision multiple access), OFDMA (orthogonal frequency division multipleaccess), SC-FDMA (single carrier frequency division multiple access) andthe like. CDMA can be implemented with such a radio technology as UTRA(universal terrestrial radio access), CDMA 2000 and the like. TDMA canbe implemented with such a radio technology as GSM/GPRS/EDGE (GlobalSystem for Mobile communications)/General Packet Radio Service/EnhancedData Rates for GSM Evolution). OFDMA can be implemented with such aradio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, E-UTRA (Evolved UTRA), etc. UTRA is a part of UMTS (UniversalMobile Telecommunications System). 3GPP (3^(rd) Generation PartnershipProject) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS)that uses E-UTRA. The 3GPP LTE adopts OFDMA in downlink (hereinafterabbreviated DL) and SC-FDMA in uplink (hereinafter abbreviated UL). And,LTE-A (LTE-Advanced) is an evolved version of 3GPP LTE.

For clarity, the present invention mainly concerns IEEE 802.11 system,by which the technical characteristic of the present invention may benon-limited.

The General of System

FIG. 1 is a diagram for an example of a structure of IEEE 802.11 systemto which the present invention is applicable.

IEEE 802.11 structure can consist of a plurality of configurationelements and a WLAN supporting mobility of an STA, which is transparentto an upper layer, can be provided by interaction of a plurality of theconfiguration elements. A basic service set (hereinafter abbreviatedBSS) may correspond to a basic configuration block in IEEE 802.11 LAN.FIG. 1 depicts an example that there exist two BSSs (BSS 1 and BSS 2)and two STAs are included in each of the BSSs as members, respectively(STA 1 and STA 2 are included in the BSS 1 and STA 3 and STA 4 areincluded in the BSS 2). An oval indicating a BSS in FIG. 1 may becomprehended as a coverage area of the STAs included in the BSS tomaintain a communication. This area can be called a basic service area(hereinafter abbreviated BSA). If an STA moves out of the BSA, the STAcannot directly communicate with different STAs within the BSA.

A BSS of a most basic type in IEEE 802.11 LAN may correspond to anindependent BSS (hereinafter abbreviated IBSS). For instance, the IBSSmay have a minimum form consisting of two STAs only. The BSS (BSS 1 orBSS 2), which is the simplest form and omitted different configurationelements, in FIG. 1 may correspond to a representative example of theIBSS. This sort of configuration is available when the STAs are able todirectly communicate with each other. And, this kind of LAN can beconfigured when a LAN is necessary instead of being configured inadvance. Hence, this network may be called an ad-hoc network.

When power of an STA is turned on or turned off or an STA enters into aBSS area or gets out of the BSS area, a membership of the STA in a BSScan be dynamically changed. In order to be a member of the BSS, the STAcan join the BSS using a synchronization process. In order to access allservices based on a BSS structure, the STA should be associated with theBSS. The association can be dynamically set and may include a use of adistribution system service (hereinafter abbreviated DSS).

FIG. 2 is a diagram for a different example of a structure of IEEE802.11 system to which the present invention is applicable. FIG. 2 is aform to which such a configuration element as a distribution system(DS), a distribution system medium (DMS), an access point (AP), and thelike is added to the structure of FIG. 1.

In a LAN, a direct distance between stations can be restricted by PHYperformance. In some cases, the distance may be sufficient to perform acommunication. Yet, in some cases, it may be necessary to perform acommunication of a longer distance between stations. The distributionsystem (DS) can be configured to support an extended coverage.

The DS means a structure that BSSs are connected with each other.Specifically, instead of independently existing as depicted in FIG. 1, aBSS may exist as a configuration element of an extended form of anetwork consisting of a plurality of BSSs.

The DS is a logical concept and can be characterized by an attribute ofthe distribution system medium (DSM). Regarding this, IEEE 802.11standard logically distinguishes a wireless medium (WM) from thedistribution system medium (DSM). Each of the logical media is used forpurposes different from each other and is used by configuration elementsdifferent from each other. According to the definition of IEEE 802.11standard, the media may be limited to neither an identical medium normedia different from each other. Flexibility of the IEEE 802.11 LANstructure can be explained in that pluralities of the media arelogically different from each other. In particular, IEEE 802.11 LANstructure can be variously implemented. The corresponding LAN structurecan be independently characterized by a physical attribute of eachimplementation example.

The DS can support a mobile device in a manner of providing the mobiledevice with a seamless integration of a plurality of BSSs and logicalservices necessary for controlling an address to a destination.

The AP enables related STAs to access the DS via the WM and means anentity having STA functionality. Data can move between the BSS and theDS via the AP. For instance, an STA 2 and an STA 3 depicted in FIG. 2have STA functionality and provide a function of enabling the relatedSTAs (an STA 1 and an STA 4) to access the DS. And, since all APsbasically correspond to an STA, all APs are entities capable of beingaddressed. An address used by the AP for a communication in the WM maynot be identical to an address used by the AP for a communication in theDS.

A data transmitted to an STA address of an AP from one of STAs relatedto the AP is always received in an uncontrolled port and can beprocessed by IEEE 802.1X port entity. And, if a controlled port isauthenticated, a transmission data (or a frame) can be delivered to theDS.

FIG. 3 is a diagram for a further different example of a structure ofIEEE 802.11 system to which the present invention is applicable. FIG. 3conceptually shows an extended service set (hereinafter abbreviated ESS)configured to provide a wider coverage in addition to the structure ofFIG. 2.

A wireless network of an arbitrary size and complexity may consist of aDS and BSSs. This kind of network is called an ESS network in IEEE802.11 system. The ESS may correspond to a set of BSSs connected with asingle DS. Yet, the ESS does not include the DS. The ESS network is seenas an IBSS network in a LLC (logical link control) layer. STAs includedin the ESS can communicate with each other and moving STAs can move fromone BSS to another BSS (within an identical ESS) in a manner of beingtransparent to the LLC.

According to IEEE 802.11, nothing is assumed for a physical location ofthe BSSs depicted in FIG. 3. Forms described in the following are allavailable in IEEE 802.11. The BSSs can be partly overlapped with eachother. This is a form generally used to provide a contiguous coverage.And, the BSSs may not be physically connected with each other and thereis no limit for a logical distance between the BSSs. The BSSs can bephysically positioned at an identical location. This can be used toprovide a redundancy. And, one (or more) IBSS or ESS networks canphysically exist in an identical space as one (or more) ESS network.This may correspond to a form of the ESS network in case that an ad-hocnetwork operates in the location at which the ESS network exists,physically duplicated IEEE 802.11 networks are configured by differentorganizations, two or more different access and security policies arerequired in an identical location, and the like.

FIG. 4 is a diagram for an example of a structure of WLAN system. FIG. 4shows an example of an infrastructure BSS including a DS.

According to the example of FIG. 4, an ESS consists of a BSS 1 and a BSS2. In a WLAN system, an STA corresponds to a device operating inaccordance with a MAC/PHY regulation of IEEE 802.11. The STA includes anAP STA and a non-AP STA. The non-AP STA corresponds to a device directlycontrolled by a user such as a laptop computer and a cellular phone. Inthe example of FIG. 4, an STA 1, an STA 3, and an STA 4 correspond tothe non-AP STA and an STA 2 and an STA 5 correspond to the AP STA.

In the following description, the non-AP STA may be called a terminal, awireless transmit/receive unit (WTRU), a user equipment (UE), a mobilestation (MS), a mobile terminal (MT), a mobile subscriber station (MSS),and the like. And, the AP is a concept corresponding to a base station(BS), a node B, an evolved Node B (eNB), a base transceiver system(BTS), a femto base station (femto BS), and the like in a differentwireless communication field.

FIG. 5 is a diagram for an example of a data link layer structure and aphysical layer structure of IEEE 802.11 system to which the presentinvention is applicable.

Referring to FIG. 5, a physical layer 520 can include a PLCP (physicallayer convergence procedure) entity 521 and a PMD (physical mediumdependent) entity 522. The PLCP entity 21 plays a role of connecting aMAC sub-layer 510 with a data frame. The PMD entity 522 plays a role oftransceiving data with two or more STAs in wireless using an OFDMscheme.

Both the MAC sub-layer 510 and the physical layer 520 can include aconceptual management entity. The conceptual management entity of theMAC sub-layer and the conceptual management entity of the physical layerare called a MLME (MAC sub-layer management entity) 511 and a PLME(physical layer management entity) 523, respectively. These entities51/521 provide a layer management service interface via an operation ofa layer management function.

In order to provide a precise MAC operation, there may exist an SME(station management entity) 530 in each STA. The SME 530 is a managemententity independent of each layer. The SME collects layer based stateinformation from a plurality of layer management entities or configuresa value of specific parameters of each layer. The SME 530 can performthe aforementioned function instead of general system managemententities and can implement a standard management protocol.

The aforementioned various entities can interact with each other usingvarious methods. FIG. 5 shows an example of exchanging a GET/SETprimitive with each other. An XX-GET.request primitive is used forrequesting a value of an MIB (management information base) attribute. Ifa state corresponds to ‘SUCCESS’, an XX-GET.confirm primitive returnsthe MIB attribute value. Otherwise, the XX-GET.confirm primitive returnsin a manner of displaying an error sign in a state field. AnXX-SET.request primitive is used to make a request for setting adesignated MIB attribute with a given value. If the MIB attribute meansas a specific operation, the request makes a request for an execution ofthe specific operation. And, if a state corresponds to ‘SUCCESS’, anXX-SET.confirm primitive means that the designated MIB attribute is setwith the requested value. Other cases except the aforementioned casesindicate an error situation. If the MIB attribute means a specificoperation, this primitive can confirm that the operation is performed.

As shown in FIG. 5, the MLME 511 and the SME 530 can exchange variousprimitives with each other via an MLME_SAP (MLME_service access point)550. And, the PLME 523 and the SME 530 can exchange various primitiveswith each other via a PLME_SAP (PLME_service access point) 560. And, aprimitive can be exchanged with each other between the MLME 511 and thePLME 523 via an MLME-PLME_SAP (MLME-PLMEservice access point) 570.

Link Setup Process

FIG. 6 is a flowchart for explaining a general link setup process in awireless LAN system to which the present invention is applicable.

In order for an STA to setup a link with a network and transceive datawith the network, first of all, the STA should discover the network,perform authentication, establish association and undergo anauthentication procedure for security. A link setup process may also becalled a session initiation process or a session setup process. And, theprocesses of the link setup process including the discovery, theauthentication, the association and the security configuration can becommonly called the association process.

An example of the link setup process is explained in the following withreference to FIG. 6.

In the step S610, an STA can perform a network discovery operation. Thenetwork discovery operation can include a scanning operation of the STA.In particular, in order for the STA to access a network, the STA shouldfind out a network in which the STA is able to participate. Before theSTA participates in a wireless network, the STA should identify acompatible network. A process of identifying a network existing at aspecific area is called a scanning.

A scanning scheme can be classified into an active scanning and apassive scanning.

For instance, FIG. 6 shows a network discovery operation including anactive scanning process. In case of performing an active scanning, inorder for an STA performing the scanning to search for APs existing inthe vicinity of the STA, the STA transmits a probe request frame whilemoving around channels and waits for a response in response to the proberequest frame. A responder transmits a probe response frame to the STA,which have transmitted the probe request frame, in response to the proberequest frame. In this case, the responder may correspond to an STA,which have lastly transmitted a beacon frame in a BSS of a channelcurrently being scanned. Since an AP transmits a beacon frame in theBSS, the AP becomes a responder. On the contrary, in an IBSS, since STAsin the IBSS transmit a beacon frame in turn, a responder varies. Forinstance, if an STA transmits a probe request frame on a #1 channel andreceives a probe response frame on the #1 channel, the STA storesBSS-related information included in the received probe response frame,moves to a next channel (e.g., a #2 channel) and may be then able toperform a scanning (i.e., transmitting/receiving a proberequest/response) using an identical method.

Although it is not depicted in FIG. 6, a scanning operation can beperformed by a passive scanning scheme. An STA performing a scanning inthe passive scanning waits for a beacon frame while moving aroundchannels. The beacon frame is one of management frames in IEEE 802.11.The beacon frame is periodically transmitted to notify existence of awireless network, make the STA performing the scanning find out thewireless network and participate in the wireless network. An AP plays arole of periodically transmitting the beacon frame in the BSS and STAsin the IBSS transmit the beacon frame in turn in the IBSS. If the STAperforming the scanning receives the beacon frame, the STA storesinformation on the BSS included in the beacon frame and records beaconframe information in each channel while moving to a different channel.Having received the beacon frame, the STA stores BSS-related informationincluded in the received beacon frame, moves to a next channel and canperform scanning using an identical method in the next channel.

When comparing the active scanning with the passive scanning, the activescanning is superior to the passive scanning in terms of a low delay anda less power consumption.

In the step S620, an authentication process can be performed after anetwork is discovered by the STA. In order to clearly distinguish theauthentication process from a security setup operation described in thefollowing in the step S640, the authentication process can be called afirst authentication process.

The authentication process includes a process that the STA transmits anauthentication request frame to the AP and the AP transmits anauthentication response frame to the STA in response to theauthentication request frame. An authentication frame used for theauthentication request/response corresponds to a management frame.

The authentication frame can include information on an authenticationalgorithm number, an authentication transaction sequence number, astatus code, a challenge text, an RSN (robust security network), afinite cyclic group and the like. The above-mentioned corresponds to apart of examples of information capable of being included in theauthentication request/response frame only. The above-mentionedinformation can be replaced with different information or additionalinformation can be further included as well.

The STA can transmit the authentication request frame to the AP. The APcan determine whether to permit authentication for the STA based oninformation included in the received authentication request frame. TheAP can provide a result of authentication process to the STA via theauthentication response frame.

After the STA is successfully authenticated, an association process canbe performed in the step S630. The association process can include aprocess that the STA transmits an association request frame to the APand the AP transmits an association response frame to the STA inresponse to the association request frame.

For instance, the association request frame can include information onvarious capabilities, information on beacon listening interval, an SSID(service set identifier), supported rates, supported channels, an RSN, amobility domain, supported operating classes, a TIM (traffic indicationmap) broadcast request, an interworking service capability and the like.

For instance, the association response frame can include information onvarious capabilities, information on a status code, an AID (associationID), supported rates, an EDC (enhanced distributed channel access)parameter set, an RCPI (received channel power indicator), an RSNI(received signal to noise indicator), a mobility domain, a timeoutinterval (association comeback time), an overlapping BSS scan parameter,a TIM broadcast response, a QoS (quality of service) map and the like.

The above-mentioned corresponds to a part of examples of informationcapable of being included in the association request/response frameonly. The above-mentioned information can be replaced with differentinformation or additional information can be further included as well.

After the STA is successfully associated with the network, a securitysetup process can be performed in the step S640. The security setupprocess in the step S640 may be called an authentication process via anRSNA (robust security network association) request/response. Or, theauthentication process in the step S620 is called a first authenticationprocess and the security setup process in the step S640 can be simplycalled an authentication process.

For instance, the security setup process in the step S640 may include aprocess of performing a private key setup via four-way handshaking viaan EAPOL (extensible authentication protocol over LAN) frame. And, thesecurity setup process may be performed according to a security schemenot defined by IEEE 802.11 standard.

Evolution of WLAN

As a lately established technological standard to overcome a limit fortransmission speed in a wireless LAN, there exist IEEE 802.11n. Anobject of IEEE 802.11n is to increase speed and reliability of a networkand enlarge management distance of the wireless network. Morespecifically, IEEE 802.11n supports HT (high throughput) where dataprocessing speed is greater than maximum 540 Mbps. And, IEEE 802.11n isbased on MIMO (multiple inputs and multiple outputs) corresponding to atechnology using multiple antennas in both a transmitting end and areceiving end to minimize a transmission error and optimize data speed.

As dissemination of a wireless LAN is vitalized and an application usingthe wireless LAN is diversified, a necessity for a new wireless LAN iscoming to the fore to support a throughput higher than the dataprocessing speed supported by IEEE 80.11n. A next generation wirelessLAN system (e.g., IEEE 802.11ac) supporting a VHT (very high throughput)corresponds to a next version of the IEEE 802.11n wireless LAN system.The system is one of IEEE 802.11 wireless LAN systems newly proposed tosupport data processing speed faster than 1 Gbps in a MAC service accesspoint (SAP).

In order to efficiently use a wireless channel, the next generationwireless LAN system supports transmission of an MU-MIMO (multi usermultiple input multiple output) scheme of which a plurality of STAsaccess a channel at the same time. According to the MU-MIMO transmissionscheme, an AP can transmit a packet to one or more MIMO-paired STAs atthe same time. And, supporting a wireless LAN operation in a whitespaceis under discussion. For instance, introduction of wireless LAN in sucha TV whitespace (TV WS) as a frequency band (e.g., 54˜698 MHz) of idlestate, which is caused by digitalization of an analog TV, is discussedas IEEE 802.11af standard. Yet, this is just an example. The white spacemay correspond to a licensed band capable of being preferentially usedby a licensed user. The licensed user indicates a user corresponding toa person that the use of the licensed band is granted. The licensed usercan be called a licensed device, a primary user, am incumbent user orthe like.

For instance, an AP and/or an STA operating in the WS should provide aprotection function to the licensed user. For instance, if such alicensed user as a microphone already uses a specific WS channel, whichis a divided frequency band according to a regulation to have a specificbandwidth in the WS band, the AP and/or the STA cannot use a frequencyband corresponding to the WS channel to protect the licensed user. And,if the licensed user uses a frequency band, which is currently used fortransmitting and/or receiving a frame, the AP and/or the STA should stopusing the frequency band.

Hence, the AP and/or the STA should preferentially perform a procedurefor figuring out whether a specific frequency band is usable in the WSband, in other word, whether there is a licensed user in the frequencyband. The procedure figuring out whether there is the licensed user inthe specific frequency band is called spectrum sensing. As a spectrumsensing mechanism, an energy detection scheme, a signature detectionscheme and the like can be utilized. If strength of a reception signalis greater than a prescribed value, it can be determined as the licenseduser is using the signal. Or, if a DTV preamble is detected, it can bedetermined as the licensed user is using the signal.

And, as a next generation communication technology, an M2M(machine-to-machine) communication technology is under discussion. Atechnology standard to support M2M communication is developing as IEEE802.11ah in the IEEE 802.11 wireless LAN system. The M2M communicationindicates a communication scheme including one or more machines and canalso be called an MTC (machine type communication) or an objectcommunication. In this case, the machine indicates an entity notrequiring a direct control or involvement of a human. For instance,examples of the machine may include such a personal device as asmartphone as well as such a device as a meter equipped with a wirelesscommunication module or an auto-vending machine. The M2M communicationcan include a communication between devices (e.g., D2D (device-to-devicecommunication)), a communication between a device and a server(application server) and the like. As an example of the communicationbetween the device and the server, there are a communication between anauto-vending machine and a server, a communication between a POS (pointof sale) device and a server, a communication between an electricity,gas or water supply meter and a server. Besides, an M2M communicationbased application may include security, transportation, healthcare andthe like. According to a characteristic of the aforementioned appliedexample, it is necessary for the M2M communication to slowly transmitand receive a less amount of data from time to time in environment inwhich very large number of devices are existing.

Specifically, the M2M communication should be able to support manynumber of STAs. According to a currently defined wireless LAN system, itis assumed a case that maximum 2007 STAs are connected to a single AP.Yet, the M2M communication is discussing methods of supporting a casethat more STAs (about 6000 STAs) are connected to a single STA. And, itis anticipated that there will be an application supporting/requiring alow transmission speed in the M2M communication. In order to smoothlysupport the application, for instance, an STA can recognize whetherthere exist data to be transmitted to the STA based on a TIM (trafficindication map) element in a wireless LAN system. Currently, methods ofreducing a bitmap size of the TIM is under discussion. And, it isanticipated that there will be great amount of traffic where atransmission/reception interval is very long in the M2M communication.For instance, it is required to transmit and receive a very small amountof data in every long period (e.g., in every one month) such as theamount of using electricity/gas/water supply. Hence, although the numberof STAs capable of being connected to a single AP considerablyincreases, methods of efficiently supporting a case that there are veryfew number of STA receiving data frame from the AP within a beaconinterval are under discussion in the wireless LAN system.

As mentioned in the foregoing description, a wireless LAN technology israpidly evolving. Besides the aforementioned examples, a technology fora direct link setup, enhancement of a media streaming capability,supporting an initial session setup of a high speed and/or a largescale, supporting an enlarged bandwidth and operating frequency and thelike is developing.

Frame Structure

FIG. 7 is a diagram for an example of a MAC frame format of IEEE 802.11system to which the present invention is applicable.

Referring to FIG. 7, a MAC frame format includes a MAC header (MHR), aMAC payload and a MAC footer (MFR). The MHR is defined by a regionincluding a frame control field, a duration/ID field, an address 1field, an address 2 field, an address 3 field, a sequence control field,an address 4 field, a QoS control field and a HT control field. A framebody field is defined by the MAC payload. Data intended to betransmitted by upper layer is positioned at the frame body field. Theframe body field has a variable size. A frame check sequence (FCS) fieldis defined by the MAC footer and is used to detect an error of the MACframe.

A minimum frame format is configured by the first three fields (theframe control field, the duration/ID field and the address 1 field) anda very last field (the FCS field). The first three fields and the lastfield exist in all frames. The remaining fields can exist in a specificframe type only.

Information included in each of the aforementioned fields may follow thedefinition of IEEE 802.11 system. And, the each of the aforementionedfields corresponds to an example of fields capable of being included ina MAC frame. Each field can be replaced with a different field or anadditional field can be further included as well.

FIG. 8 is a diagram for an example of an HT format of an HT controlfield in a MAC frame according to FIG. 7.

Referring to FIG. 8, the HT control field can include a VHT subfield, alink adaptation subfield, a calibration position subfield, a calibrationsequence subfield, a channel state information (CSI)/steering subfield,an NDP (null data packet) announcement subfield, an AC (access category)constraint subfield, an RDG (reverse direction grant/more) PPDU subfieldand a reserved subfield.

The link adaptation subfield can include a training request (TRQ)subfield, an MAI (MCS (modulation and coding scheme) request or an ASEL(antenna selection) indication) subfield, an MCS feedback sequenceindication (MFSI) subfield, an MCS feedback and antenna selectioncommand/data (MFB/ASELC) subfield.

If a sounding PPDU is requested to a responder, the TRQ subfield is setto 1. If the sounding PPDU is not requested to the responder, the TRQsubfield is set to 0. And, if the MAI subfield is set to 14, itindicates an antenna selection indication (ASEL indication) and theMFB/ASELC subfield is interpreted by the antenna selection command/data.Otherwise, the MAI subfield indicates an MCS request and the MFB/ASELCsubfield is interpreted by an MCS feedback. When the MAI subfieldindicates an MCS request (MRO), if MCS feedback is not requested, theMAI subfield is set to 0. If the MCS is requested, the MAI subfield isset to 1. The sounding PPDU indicates a PPDU delivering a trainingsymbol usable for a channel estimation.

The aforementioned each of the subfields corresponds to an example ofsubfields capable of being included in the HT control field. Each fieldcan be replaced with a different subfield. Or, an additional subfieldcan be further included.

FIG. 9 is a diagram for an example of a VHT format of an HT controlfield in a MAC frame according to FIG. 7.

Referring to FIG. 9, the HT control field can include a VHT subfield, anMRO subfield, an MSI subfield, an MCS feedback sequence indication/groupID lowest bit (MFSI/GID-L: LSB of group ID) subfield, an MFB subfield, agroup ID highest bit (GID-H: MSB of group ID) subfield, a coding typesubfield, an MFC response transmission type (FB Tx type: transmissiontype of MFB response) subfield, an unsolicited MFB subfield, an ACconstraint subfield, an RDG/more PPDU subfield. And, the MFB subfieldcan include a VHT space-time stream number (N_STS: number of space timestreams) subfield, an MCS subfield, a bandwidth (BW) subfield and asignal to noise ratio (SNR) subfield.

Table 1 shows explanation on each subfield in a VHT format of the HTcontrol field.

TABLE 1 Subfield Meaning Definition MRQ MCS request If MCSfeedback(solicited MFB) is requested, set to 1. Otherwise, set to 0. MSIMRO sequence If MRO subfield is set to 1, MSI identifier subfieldincludes sequence number within a scope ranging from 0 to 6 identifyinga specific request. If MRO subfield is set to 0, MSI subfield isreserved. MFSI/ MFB sequence If unsolicited MFB subfield is set to 0,GID-L identifier/ MFSI/GID-L subfield includes a LSB of reception valueof MSI included in a group ID frame indicated by MFB information. Ifunsolicited MFB subfield is set to 1, MFSI/GID-L subfield includeslowest 3 bits of a group ID of PPDU indicated by solicited MFB. MFB VHTN_STS, MFB subfield includes a recommended MCS, BW, MFB. MCS = 15, VHTN_STS = 7 SNR feedback indicate that there is no feedback. GID-H MSB ofIf unsolicited MFB subfield is set to 1, group ID GID-H subfieldincludes highest 3 bits of a group ID of PPDU indicated by theunsolicited MFB. Coding type Coding type of If unsolicited MFB subfieldis set to 1, MFB response coding type subfield includes 1 in case ofcoding information (BCC (binary convolution code)) indicated by theunsolicited MFB, 0 in case of LDPC (low-density parity check).Otherwise, reserved. FB Tx type Transmission If unsolicited MFB subfieldis set to 1 type of MFB and FB Tx type subfield is set to 0, theresponse unsolicited MFB indicates either unbeamformed VHT PPDU ortransmit diversity using STBC (space-time block coding) VHT PPDU. Ifunsolicited MFB subfield is set to 1 and FB Tx type subfield is set to1, the unsolicited MFB indicates beamformed SU-MIMO (single user MIMO)VHT PPDU. Otherwise, reserved. Unsolicited Unsolicited If MFB is not aresponse of MRQ, set to MFB MCS feedback 1. If MFB is a response of MRQ,set to indicator 0. Ac constraint If response for reverse directiongrant (RDG) includes data frame from a traffic identifier (TID), set to0. If response for reverse direction grant (RDG) includes a framereceived from AC identical to last data frame received from an identicalreverse direction (RD) initiator only, set to 1. RDG/more When RDG/morePPDU subfield PPDU corresponds to 0, if reverse direction (RD) initiatortransmits, it indicates there is no reverse direction grant (RDG). Ifreverse direction (RD) responder transmits, it indicates PPDU deliveringMAC frame is last transmission. When RDG/more PPDU subfield correspondsto 1, if reverse direction (RD) initiator transmits, it indicates thereexists reverse direction grant (RDG). If reverse direction (RD)responder transmits, there exist following different PPDU after PPDUdelivering MAC frame.

The aforementioned each of the subfields corresponds to an example ofsubfields capable of being included in the HT control field. Each fieldcan be replaced with a different subfield. Or, an additional subfieldcan be further included.

In the meantime, the MAC sub-layer delivers an MAC protocol data unit(MPDU) to a physical layer as a physical service data unit (PSDU). APCCP entity adds a physical header and a preamble to the received PSDUand generates a PLCP protocol data unit (PPDU).

FIG. 10 is a diagram for an example of a PPDU frame format of IEEE802.11n system to which the present invention is applicable.

FIG. 10 (a) shows an example of a PPDU frame according to a non-HTformat, an HT mixed format and an HT-greenfield format.

The non-HT format indicates a frame format for a legacy system (IEEE802.11a/g) STA. A non-HT format PPDU includes a legacy format preambleconsisting of a legacy-short training field (L-STF), a legacy-longtraining field (L-LTF) and a legacy-signal (L-SIG) field.

The HT mixed format permits a communication with a legacy system STA andindicates a frame format for IEEE 802.11n STA at the same time. The HTmixed format PPDU includes a legacy format preamble consisting of theL-STF, the L-LTF and the L-SIG and an HT format preamble consisting ofan HT-short training field (HT-STF), an HT-long training field (HT-LTF)and an HT-signal (HT-SIG) field. Since the L-STF, the L-LTF and theL-SIG mean legacy fields for backward compatibility, a part from theL-STF to the L-SIG is identical to the non-HT format. An STA canidentify the mixed format PPDU using the HT-SIG field appearing afterthe part.

The HT-greenfield format is a format not compatible with a legacysystem. The HT-greenfield format indicates a format used for an IEEE802.11n STA. an HT-greenfield format PPDU includes a greenfield preambleconsisting of an HT-greenfield-STF (HT-GF-STF), an HT-LTF1, an HT-SIGand one or more HT-LTFs.

A data field includes a service field, PSDU, tail bit and pad bit. Allbits of the data field are scrambled.

FIG. 10 (b) shows the service field included in the data field. Theservice field has 16 bits. Each bit is numbered by 0 to 15. Each bit issequentially transmitted from a bit #0. The bit #0 to a bit #6 are setto 0 and used to synchronize a descrambler installed in a transmittingend.

FIG. 11 is a diagram for an example of a VHT PPDU frame format of IEEE802.11ac system to which the present invention is applicable.

Referring to FIG. 11, a VHT format PPDU includes a legacy formatpreamble consisting of L-STF, L-LTF and L-SIG and a VHT format preambleconsisting of VHT-SIG-A, HT-STF and HT-LTF before a data field. Sincethe L-STF, the L-LTF and the L-SIG mean a legacy field for backwardcompatibility, a part from the L-STF to the L-SIG is identical to thenon-HT format. An STA can identify the VHT format PPDU using the VHT-SIGfield appearing after the part.

The L-STF is a field used for frame detection, auto gain control (AGC)diversity detection, coarse frequency/time synchronization and the like.The L-LTF is a field used for fine frequency/time synchronization,channel estimation and the like. The L-SIG is a field used fortransmitting legacy control information. The VHT-SIG-A is a VHT fieldused for transmitting control information included in VHT STAs incommon. The VHT-STF is a field used for AGC for MIMO and a beamformedstream. The VHT-LTFs is a field used for channel estimation for MIMO anda beamformed stream. The VHT-SIG-B is a field used for transmittingspecified control information.

Medium Access Mechanism

In a wireless LAN system according to IEEE 802.11, a basic accessmechanism of MAC (medium access control) corresponds to a CSMA/CA(carrier sense multiple access with collision avoidance) mechanism. TheCSMA/CA mechanism is also called a DCF (distributed coordinationfunction) of IEEE 802.11 MAC. Basically, “listen before talk” accessmechanism is adopted. According to this sort of access mechanism, an APand/or an STA can perform CCA (clear channel assessment) to sense aradio channel or medium during a prescribed time interval (e.g., DIFS(DCF inter-frame space) prior to beginning of transmission. As a resultof sensing, if it is determined that the medium is in an idle status, aframe is transmitted using the corresponding medium. On the contrary, ifit is detected as the medium is in an occupied status, the AP and/or theSTA does not start transmission of its own, waits for accessing themedium in a manner of configuring a delay period (e.g., random backoffperiod) and may be then able to attempt frame transmission after thedelay period. When the random backoff period is applied, since it isexpected that many STAs attempt the frame transmission after waiting fortime period different from each other, collision can be minimized.

And, IEEE 802.11 MAC protocol provides an HCF (hybrid coordinationfunction). The HCF is performed based on the DCF and a PCF (pointcoordination function). The PCF is a polling based synchronous accessscheme. In order to make all receiving Aps and/or STAs receive a dataframe, the data frame is periodically polled. And, the HCF includes anEDCA (enhanced distributed channel access) and an HCCA (HCF controlledchannel access). The EDCA is a contention based access scheme used by aprovider to provide a data frame to a plurality of users. The HCCA is anon-contention based channel access scheme using a polling mechanism.And, the HCF includes a medium access mechanism used for improving QoS(quality of service) of WLAN and can transmit QoS data in both CP(contention period) and CFP (contention free period).

FIG. 12 is a diagram for explaining a backoff process in a wireless LANsystem to which the present invention is applicable.

An operation based on a random backoff period is explained withreference to FIG. 12.

If a medium in an occupied or busy status changes its status to an idlestatus, many STAs can attempt transmission of a data (or a frame). Inthis case, as a method of minimizing a collision, each of the STAsselects a random backoff count, waits for time as long as slot timecorresponding to the random backoff count and may be then able toattempt the transmission. The random backoff count has a pseudo-randominteger value and can be determined by one among values ranging from 0to CW. In this case, the CW is a contention window parameter value. ACWmin is given as an initial value of the CW parameter. If transmissionfails (e.g., ACK for a transmitted frame is not received), a doubledvalue can be given for the CW parameter. If the CW parameter valuebecomes CWmax, data transmission can be tried out until the datatransmission is successful while the CWmax value is maintained. When thedata transmission is successful, the CW parameter is reset to the CWminvalue. It is preferable to configure the CW, the CEmin and the CWmaxwith 2n−1 (n=0, 1, 2, . . . ).

Once a random backoff process starts, an STA continuously monitors amedium while a backoff slot is count downed according to a determinedbackoff count value. If the medium is monitored as in an occupiedstatus, countdown is stopped and waits. When the medium is in an idlestatus, remaining countdown is resumed.

According to an example shown in FIG. 12, when a packet to betransmitted arrives at a MAC of an STA 3, the STA 3 checks that a mediumis in an idle status as long as a DIFS and may be then able to directlytransmit a frame. Meanwhile, remaining STAs monitor that the medium isin an occupied (busy) status and standby. In this case, data to betransmitted may occur in each of an STA 1, an STA 2 and an STA 5. If itis monitored that the medium is in the idle status, each of the STAswaits for time as long as the DIFS and may be then able to performcountdown of a backoff slot according to a random backoff count valueselected by each of the STAs. According to the example shown in FIG. 12,the STA 2 has selected a smallest backoff count value and the STA 1 hasselected a largest backoff count value. In particular, the example ofFIG. 12 shows a case that a residual backoff time of the STA 5 isshorter than a residual backoff time of the STA 1 when the STA 2finishes the backoff count and starts frame transmission. The STA 1 andthe STA 5 temporarily stop countdown and standby while the STA 2 isoccupying the medium. If the occupation of the STA 2 ends and the mediumis back in the idle status, the STA 1 and the STA 5 wait for time aslong as the DIFS and then resume the backoff count. In particular, theSTA 1 and the STA 5 countdown remaining backoff slot as long as theresidual backoff time and may be then able to start the frametransmission. Since the residual backoff time of the STA 5 is shorterthan the residual backoff time of the STA 1, the STA 5 starts the frametransmission. Meanwhile, data to be transmitted may also occur in theSTA 4 while the medium is occupied by the STA 2. In this case, if themedium is in the idle status, the STA 4 waits for time as long as theDIFS and may be then able to performs countdown according to a randombackoff count value selected by the STA 4 and start the frametransmission. The example of FIG. 12 shows a case that the residualbackoff time of the STA 5 is coincidentally matched with the randombackoff count value of the STA 4. In this case, a collision may occurbetween the STA 4 and the STA 5. If the collision occurs, since both theSTA 4 and the STA 5 cannot receive ACK, data transmission fails. In thiscase, the STA 4 and the STA 5 select a random backoff count value afterenlarging the CW value as much as twice and can perform the countdown.Meanwhile, the STA 1 stand by while the medium is occupied by the STA 4and the STA 5 due to the transmission. If the medium is back in the idlestatus, the STA 1 waits for time as long as the DIFS and may be thenable to perform the frame transmission when the residual backoff timeelapses.

Sensing Operation of STA

As mentioned in the foregoing description, the CSMA/CA mechanismincludes a physical carrier sensing directly sensed by the AP and/or theSTA. besides the physical carrier sensing, the CSMA/CA mechanism alsoincludes a virtual carrier sensing. The virtual carrier sensing isconfigured to compensate a problem capable of being occurred in a mediumaccess such as a hidden node problem. In order to perform the virtualcarrier sensing, a MAC of a wireless LAN system may use a networkallocation vector (NAV). The NAV is a value used for indicatingremaining time to an available status of a medium, indicated by an APand/or an STA currently using the medium or having authority of usingthe medium for a different AP and/or an STA. Hence, a value configuredas the NAV corresponds to a duration for which the use of the medium isreserved by the AP and/or the STA transmitting a corresponding frame.The STA receiving the NAV cannot access the media for the duration. Forinstance, the NAV can be configured according to a value of a durationfield of a MAC header of a frame.

And, in order to reduce possibility of collision, a robust collisiondetect mechanism has been introduced. Regarding this, it shall bedescribed with reference to FIG. 13 and FIG. 4. Although an actualcarrier sensing range and a transmission range may not be identical toeach other, for clarity, assume that they are identical to each other.

FIG. 13 is a diagram for explaining a hidden node and an exposed node.

FIG. 13 (a) shows an example of a hidden node. The example shows a casethat an STA A is communicating with an STA B and an STA C hasinformation to be transmitted. Specifically, although the example showsa situation that the STA A transmits information to the STA B, beforedata is transmitted to the STA B by the STA C, it may be determined as amedium is in an idle status when a carrier sensing is performed. This isbecause transmission (i.e., occupation of the medium) of the STA A maynot be sensed in a position of the STA C. In this case, since the STA Breceives information of the STA A and information of the STA C at thesame time, a collision occurs. In this case, the STA A may be consideredas a hidden node of the STA C.

FIG. 13 (b) shows an example of an exposed node. In a situation that anSTA B is transmitting data to an STA A, the example shows a case that anSTA C has data to be transmitted from an STA D. In this case, if the STAC performs a carrier sensing, it may be determined as a medium isoccupied due to the transmission of the STA B. Hence, although the STA Chas data to transmit to the STA D, since the medium is sensed as anoccupied status, the STA C should wait until the medium becomes the idlestatus. Yet, since the STA A is positioned at the outside of atransmission range of the STA C, transmission from the STA C andtransmission from the STA B may not be collided with each other in termsof the STA A. Hence, the STA C unnecessarily waits until the STA B stopsthe transmission. In this case, the STA C may be considered as anexposed node of the STA B.

FIG. 14 is a diagram for explaining an RTS and a CTS.

In order to efficiently use a collision avoidance mechanism in asituation shown in the example of FIG. 13, such a short signaling packetas RTS (request to send), CTS (clear to send) and the like can be used.The RTS/CTS between two STAs can make a neighboring STA(s) overhear. Bydoing so, the neighboring STA(s) can consider whether information istransmitted between the two STAs. For instance, if an STA intending totransmit data transmits an RTS frame to an STA receiving the data, theSTA receiving the data transmits a CTS frame to neighboring UEs toinform that the STA receiving the data will receive the data.

FIG. 14 (a) shows an example for a method of solving a hidden nodeproblem and assume a case that both the STA A and the STA C intend totransmit data to the STA B. If the STA A transmits the RTS to the STA B,the STA B transmits the CTS to all STAs around the STA B including theSTA A and the STA C. As a result, the STA C waits until the datatransmission of the STA A and the STA B ends, thereby avoiding acollision.

FIG. 14 (b) shows an example for a method of solving an exposed nodeproblem. By making the STA C overhear RTS/CTS transmission between theSTA A and the STA B, it is able to determine that a collision will notoccur although the STA C transmits data to a different STA (e.g., STAD). In particular, the STA B transmits the RTS to all UEs around the STAB and the STA A, which actually has data to be transmitted, transmitsthe CTS. Since the STA C has received the RTS only and has not receivedthe CTS of the STA A, the STA C is able to know that the STA A ispositioned at the outside of carrier sensing of the STC C.

Power Management

As mentioned in the foregoing description, channel sensing should beperformed before an STA performs transmission and reception in awireless LAN system. Yet, sensing a channel all the time causes constantpower consumption of the STA. There is no big difference between powerconsumption in a receiving state and power consumption in a transmittingstate. Hence, if the receiving state is continuously maintained, itbecomes big burden on an STA operating with a limited power (i.e.,battery operated STA). Hence, if the STA maintains a state of receptionstandby to consistently sense a channel, it leads the STA toinefficiently consume power without any special benefit in terms of awireless LAN processing ratio. In order to solve the above-mentionedproblem, the wireless LAN system supports a power management (PM) modeof the STA.

The power management mode of an STA is divided into an active mode and apower save (PS) mode. An STA basically operates in the active mode. AnSTA operating in the active mode maintains an awake state. The awakestate corresponds to a state in which such a normal operation as frametransmission and reception, channel scanning and the like is feasible.Meanwhile, an STA operating in the PS mode operates in a manner ofswitching between a sleep state and the awake state. The STA operatingin the sleep state operates with minimum power and performs neitherframe transmission/reception nor channel scanning.

Since an STA consumes less power in a sleep state, if an STA operates inthe sleep state as long as possible, an operating period of the STAincreases. Yet, since it is impossible to transmit and receive a framein the sleep state, it is not possible for the STA to unconditionallyoperate in the sleep state. If the STA operating in the sleep state hasa frame to transmit to an AP, the STA can transmit the frame in a mannerof switching to the awake state from the sleep state. Meanwhile, if theAP has a frame to transmit to the STA, the STA operating in the sleepstate cannot receive the frame and cannot even know whether there existsthe frame to receive. Hence, in order for the STA to know whether thereexist a frame to be transmitted to the STA (or, to receive the frame ifexists), it may be necessary for the STA to switch to the awake statewith a specific period.

FIG. 15 is a flowchart for explaining a power management operation.

Referring to FIG. 15, an AP 210 transmits a beacon frame to STAs in aBSS with a prescribed interval [S211, S212, S213, S214, S215 and S216].The beacon frame includes a TIM (traffic indication map) informationelement. The TIM information element includes information indicatingwhether there exists buffered traffic for STAs connected to the AP 210and whether a frame is to be transmitted by the AP. The TIM element canbe classified into a TIM used for indicating a unicast frame and a DTIM(delivery traffic indication map) used for indicating a multicast or abroadcast frame.

The AP 210 can transmit one DTIM whenever 3 beacon frames aretransmitted.

An STA 1 220 and an STA 2 230 are STAs operating in the PS mode. The STA1 220 and the STA 230 can be configured to receive the TIM elementtransmitted by the AP 210 in a manner of switching to the awake statefrom the sleep state in every wakeup interval of a prescribed period.Each of the STAs can calculate timing of switching to the awake statebased on its own local clock. In an example of FIG. 15, assume that aclock of an STA is matched with a clock of an AP.

For instance, the prescribed wakeup interval can be configured to makethe STA 1 220 receive the TIM element in every beacon interval in amanner of being switched to the awake state. Hence, when the AP 210transmits a first beacon frame [S211], the STA 1 220 can switch to theawake state [S221]. The STA 1 220 receives the beacon frame and canobtain the TIM element. If the obtained TIM element indicates that thereexists a frame to be transmitted to the STA 1 220, the STA 1 220 cantransmit a PS-poll (power save-poll) frame, which makes a request forthe AP 210 to transmit a frame, to the AP 210 [S221 a]. The AP 210 cantransmit the frame to the STA 1 220 in response to the PS-poll frame.Having received the frame, the STA 1 220 operates in a manner ofswitching to the sleep state again.

When the AP 210 transmits a second beacon frame, since a medium is in astate of being occupied by a different device (busy medium), the AP 210cannot transmit the beacon frame in accordance with an accurate beaconinterval and may transmit the beacon frame on a delayed timing [S212].In this case, although the STA 1 220 switches the operation mode intothe awake mode according to a beacon interval, since the STA 1 is unableto receive the beacon frame transmitted by being delayed, the STA 1switches to the sleep state again [S222].

When the AP 210 transmits a third beacon frame, the beacon frame mayinclude a TIM element configured as a DTIM. Yet, since the medium is inthe state of being occupied (busy medium), the AP transmits the beaconframe in a manner of delaying the transmission [S213]. The STA 1 220operates in a manner of switching to the awake state in accordance withthe beacon interval and can obtain the DTIM via the beacon frametransmitted by the AP 210. Assume a case that the DTIM obtained by theSTA 1 220 indicates that there is no frame to be transmitted to the STA1 220 and there exists a frame for a different STA. In this case, theSTA 1 220 checks that there is no frame to be transmitted to the STA 1and may operate in a manner of switching back to the sleep state. Havingtransmitted the beacon frame, the AP 210 transmits a frame to acorresponding STA [S232].

The AP 210 transmits a fourth beacon frame [S214]. Yet, since the STA 1220 was unable to obtain information indicating that there is nobuffered traffic for the STA 1 via the TIM element which is previouslyreceived twice, the STA 1 can adjust an wakeup interval to receive theTIM element. Or, if signaling information used for adjusting an wakeupinterval value of the STA 1 220 is included in the beacon frametransmitted by the AP 210, the wakeup interval value of the STA 1 220can be adjusted. According to the present example, the STA 1 220 can beconfigured to switch a management state from a state of switching themanagement state to receive the TIM element in every beacon interval toa state of switching the management state to the wakeup state in every 3beacon intervals. Hence, since the STA 1 220 maintains the sleep statewhen the AP 210 transmits the fourth beacon frame [S214] and a fifthbeacon frame [S215], the STA 1 cannot obtain a corresponding TIMelement.

When the AP 210 transmits a sixth beacon frame [S216], the STA 1 220operates in a manner of switching to the awake state and can obtain aTIM element included in the beacon frame [S224]. Since the TIM elementcorresponds to a DTIM indicating that there exists a broadcast frame,the STA 1 220 can receive a broadcast frame transmitted by the AP 210instead of transmitting a PS-poll frame to the AP 210 [S234]. Meanwhile,an wakeup interval set to an STA 2 230 can be configured by a periodlonger than a period of the STA 1 220. Hence, the STA 2 230 switches tothe awake state on a timing that the AP 210 transmits the fifth beaconframe [S215] and can obtain the TIM element [S241]. The STA 2 230 knowsthere exists a frame to be transmitted to the STA 2 via the TIM elementand can transmit the PS-poll frame to the AP 210 to make a request forframe transmission. The AP 210 can transmit a frame to the STA 2 230 inresponse to the PS-poll frame [S233].

In order to manage such a mode as the power saving mode mentionedearlier in FIG. 15, the TIM element can include a TIM indicating whetherthere exists a frame to be transmitted to an STA or a DTIM indicatingwhether there exist a broadcast/multicast frame. The DTIM can beimplemented by a field configuration of the TIM element.

FIGS. 16 to 18 are diagrams for explaining an operation of an STA, whichhas received a TIM, in detail.

Referring to FIG. 16, an STA switches to an awake state from a sleepstate to receive a beacon frame including a TIM from an AP and can knowthere exists a buffered traffic to be transmitted to the STA byinterpreting the TIM element. The STA performs contending with differentSTAs to access a medium used for transmitting a PS-poll frame and may bethen able to transmit the PS-poll frame to the AP to make a request fordata frame transmission. Having received the PS-poll transmitted by theSTA, the AP can transmit a frame to the STA. the STA receives a dataframe and can transmit a confirmation response (ACK) in response to thedata frame. Subsequently, the STA can switch back to the sleep stateagain.

As shown in FIG. 16, having received the PS-poll frame from the STA, theAP can operate according to an immediate response scheme correspondingto a scheme transmitting a data frame in a prescribed time (e.g., SIFS(short inter-frame space) after the PS-poll reception. Meanwhile, afterthe AP received the PS-poll frame, if a data frame to be transmitted tothe STA is not ready for the SIFS time, the AP may operate in accordancewith a deferred response scheme. Regarding this, it shall be describedwith reference to FIG. 17.

In an example of FIG. 17, similar to the operations mentioned in FIG.16, the STA switches to the awake state from the sleep state, receivethe TIM from the AP and transmit the PS-poll frame to the AP byperforming contending. Despite receiving the PS-poll frame, if the AP isnot ready for a data frame during the SIFS, the AP can transmit an ACKframe to the STA instead of the data frame. After the ACK frame istransmitted, if the data frame is ready, the AP can transmit the dataframe to the STA after contending is performed. The STA transmits an ACKframe indicating that the data frame is successfully received to the APand may be then able to switch to the sleep state.

FIG. 18 shows an example of transmitting a DTIM by the AP. In order forSTAs to receive a beacon frame including a DTIM element from the AP, theSTAs can switch to the awake state from the sleep state. The STAs areable to know that a multicast/broadcast frame will be transmitted viathe received DTIM. Having transmitted the beacon frame including theDTIM, the AP can immediately transmit a data (i.e., multicast/broadcastframe) without an operation of transmitting and receiving a PS-pollframe. Having received the beacon frame including the DTIM, the STAsreceive data while maintaining the awake state and can switch back tothe sleep state after the data reception is completed.

According to the method of managing the power saving mode, which isbased on the TIM (or DTIM) mentioned earlier with reference to FIG. 16to FIG. 18, STAs can check whether there exists a data frame to betransmitted to the STAs via STA identification information included inthe TIM element. The STA identification information may correspond toinformation related to an AID (association identifier), which is anidentifier allocated to an STA when the STA is associated with an AP.

The AID is used as a unique identifier for each STA within a BSS. As anexample, according to a current wireless LAN, the AID can be allocatedby one of values ranging from 1 to 2007. According to a currentlydefined wireless LAN system, 14 bits can be allocated for the AID in aframe transmitted by an AP and/or an STA. Although the AID value can beallocated up to 16383, values ranging from 2008 to 16383 are configuredas reserved values.

Power Management Using Automatic Power Saving Delivery

Besides the power saving method based on the aforementioned PS-poll,IEEE 802.11e system provides an APSD (automatic power saving delivery)method. The APSD can be mainly classified into a scheduled APSD (s-APSD)method and an unscheduled APSD (u-APSD). The u-APSD indicates amechanism delivering a downlink frame by an STA supporting the APSDwhile an AP supporting the APSD operates in a power saving mode in amanner of switching back and forth between an awake state and a sleep(doze) state.

A QoS (quality of service) AP capable of supporting the APSD can signalsuch capability as the aforementioned to an STA using a beacon, a proberesponse, an APSD subfield of a capability information field in a(re)association response management frame.

In order for STAs to receive a part or all of a bufferable unit (BU) ofthe STAs delivered from an AP during an unscheduled service period(hereinafter abbreviated, u-SP), the STAs can use the U-APSD. If theu-SP is not in progress, the u-SP can be started in a manner that an STAtransmits a QoS data belonging to an access category (AC) configured astrigger-enabled or a QoS Null frame to the AP. In this case, atransmitted uplink frame is called a trigger frame. An aggregated MPDU(A-MPDU) includes one or more trigger frames. The unscheduled SP isterminated after the AP attempts to transmit an AC capable of beingdelivered and at least one BU to the corresponding STA. Yet, if amaximum service period length field (MAX SP length field) of a(re)association request frame of the corresponding STA has a value whichis not 0, the value of the field is limited to within an indicatedvalue.

In order to receive the BU from the AP during the u-SP, the STAdesignates one or more delivery-enabled ACs and trigger-enabled ACs ofthe STA. In order to provide QoS, IEEE 802.11e system defines 8priorities different from each other and 4 ACs based on the priorities.The STA can configure the AP to use the U-APSD using two methods. Firstof all, the STA can configure an individual U-APSD flag bit in a QoSinfo subfield of a QoS capability element delivered in a (re)associationrequest frame. If the U-APSD flag bit corresponds to 1, it indicatesthat a corresponding AC is a delivery-enabled AC and a trigger-enabledAC. If all 4 U-APSD flag subfields in the (re)association request framecorrespond to 1, all ACs related to the STA can be delivered andtriggered during the (re)association. If all 4 U-APSD flag subfields inthe (re)association request frame correspond to 0, all ACs related tothe STA can be delivered during the (re)association and atrigger-enabled AC does not exist. Or, the STA can designate one or moretrigger-enabled ACs and delivery-enabled ACs by transmitting a schedulesubfield configured by 0 in a traffic stream (TS) information field ofan ADDTS (add traffic stream) request frame including an APSD subfieldconfigured by 1 according to an AC to the AP. It is able to put an APSDconfiguration in a TSPEC request before a static U-APSD configurationdelivered in the QoS capability element. In other word, a U-APSDconfiguration of a prescribed previous AC can be overwrote with theTSPEC request. The corresponding request can be transmitted for an ACwhere an ACM subfield corresponds to 0.

The STA can configure a trigger-enabled AC or a delivery-enabled AC in amanner of configuring a TSPEC including an APSD subfield and a schedulesubfield configured by 1 and 0, respectively in uplink or downlinkdirection. An uplink TSPEC, a downlink TSPEC or a bidirectional TSPECincluding the APSD subfield configured by 1 and the schedule subfieldconfigured by 0 can configure the trigger-enabled AC and thedelivery-enabled AC. An uplink TSPEC, a downlink TSPEC or abidirectional TSPEC including an APSD subfield configured by 0 and theschedule subfield configured by 0 can configure a trigger-disabled ACand a delivery-disabled AC.

A scheduled service period (hereinafter abbreviated, s-SP) starts with afixed time interval specified in a service interval field. If an accesspolicy controls a channel access, in order for an STA to use the s-SPfor TS, the STA can transmit an ADDTS request frame including an APSDsubfield configured by 1 of a TS info field in a TSPEC element to an AP.On the contrary, if the access policy supports a contention-basedchannel access, in order to use the s-SP for the TS, the STA cantransmit an ADDTS request frame including the APSD subfield configuredby 1 and a schedule subfield of the TS info field in the TSPEC elementto the AP. If the APSD mechanism is supported by the AP and the APaccepts the corresponding ADDTS request frame transmitted by the STA,the AP may respond with an ADDTS response frame including a scheduleelement indicating that a requested service is able to be provided bythe AP. If lowest 4 octets of a timing synchronization function areidentical to a value specified in a service start time field, an initials-SP starts. An STA using the s-SP can initially wake up to receive a BUindividually addressed buffered for the STA and/or a polled BU from theAP or a hybrid coordinator (HC). Thereafter, the STA can wake up withprescribed time interval identical to a service interval (SI). The APcan adjust service start time via a successful ADDTS response frame (aresponse for an ADDTS request frame) and a schedule element of aschedule frame (transmitted on a different timing).

The s-SP starts on the service start time indicated in a scheduleelement transmitted in response to TSPEC and wakeup time scheduled inresponse to SI. Thereafter, the STA wakes up on timing according toFormula 9 in the following.

(TSF service start time)mod minimum SI=0  [Formula 1]

If an s-SP period is supported in a BSS, an STA can use both U-APSD andS-APSD for ACs different from each other on an identical time. If ascheduled delivery for an AC is set to the STA, an AP does not transmita BU, which uses the corresponding AC, during SP initialized by atrigger frame and does not process a BU using an AC received from theSTA by the trigger frame. The AP does not reject any ADDTS request frameindicating to use both the S-APSD and the U-APSD, which are configuredto use an identical AC on an identical time. APSD can be used fordelivering an individually addressed BU only. Delivering a BU addressedby group may follow a frame delivery rule used for a group-addressed BU.

A non-AP STA using the U-APSD may not receive all frames transmittedfrom an AP during a service period due to interference observed in thenon-AP STA. In this case, although identical interference is notobserved, the AP can determine that the non-AP STA does not properlyreceive a frame. A U-APSD coexistence capability enables the non-AP STAto indicate transmission duration, which is requested to be used foru-SP, to the AP. By using the transmission duration, the AP can transmita frame during the SP and the non-AP STA can improve possibility ofreceiving a frame in an interfered situation. The U-APSD coexistencecapability decreases possibility for the AP not to properly receive aframe during a service period.

FIG. 19 is a diagram for an example of a U-APSD coexistence elementformat.

Referring to FIG. 19, an element ID field is identical to a U-APSDcoexistence value. A length of an optional sub-element existing at 12added to a value of a length field. A value, which is not 0, in a TSF 0offset field indicates the number of microseconds after time (TSF time0) on which a non-AP STA is aware that interference has begun. An APuses the TSF 0 offset field together with an interval/duration field fora transmission to the non-AP STA.

An STA including a value of which‘dot1MgmtOptionUAPSDCoexistenceActivated” is ‘true’ is defined as an STAsupporting U-APSD coexistence. In this case, the STA including the valueof which ‘dot1MgmtOptionUAPSDCoexistenceActivated” is ‘true’ configuresa U-APSD coexistence field (APSD coexistence field) by 1. Otherwise, thefield is configured by 0. (If it is previously notified to both an APand a non-AP STA that U-APSD coexisting capability is supported), thenon-AP STA associated with the STA can transmit an ADDTS request frameincluding a U-APSD coexistence element to the AP.

Content of the ADDTS request frame not including the U-APSD coexistingelement is called a base ADDTS request in the following description. Ifthe ADDTS request frame is successfully received, the AP processescontent of the base ADDTS request frame. If the AP determines that thebase ADDTS request frame is not able to be approved, the AP does notprocess the U-APSD coexisting element. On the contrary, if the APdetermines that the base ADDTS request frame is able to be approved, theAP processes the U-APSD coexisting element. If the AP supports frametransmission during a U-APSD service period for a value of durationspecified in an interval/duration field of the U-APSD coexistingelement, the AP can approve the ADDTS request. Otherwise, the AP mayreject the ADDTS request.

When the AP previously approved an ADDTS request including the U-APSDcoexistence, the non-AP STA continuously using a QoS service, which isprovided by the ADDTS request frame not including the U-APSDcoexistence, can terminate the use of the U-APSD coexistence in a mannerof transmitting the ADDTS request frame not including the U-APSDcoexistence element. If the non-AP STA intends to terminate the use ofall QoS services provided by the ADDTS request frame including theU-APSD coexistence, the non-AP STA can transmit a DELTS (delete trafficstream) request frame to the AP.

If a previous ADDTS request frame is invalidated by a last successfullyreceived ADDTS request frame, the non-AP STA can transmit multiple ADDTSrequest frames to the AP. The AP supporting the U-APSD coexistence andaccepting an ADDTS request can limit U-APSD coexistence service periodaccording to a parameter specified in the U-APSD coexisting element ofthe ADDTS frame. And, the AP transmits a frame to make a request for thenon-AP STA in accordance with a rule in the following.

First of all, if the non-AP STA specifies a TSF 0 offset value by avalue which is not 0 in the U-APSD coexistence element, the AP does nottransmit a frame to the non-AP STA in the outside of the U-APSDexistence service period. The U-APSD existence service period startswhen the AP receives a U-APSD trigger frame and ends after atransmission period specified by Formula 10 in the following.

End of transmission period=T+(Interval((TTSF0Offset)modInterval)  [Formula 2]

In Formula 2, T indicates time of receiving the U-APSD trigger frame bythe AP. And, interval indicates a firstly appearing value among aduration/interval field value of the U-APSD coexistence element andtiming on which an EOSP (end of service period) bit configured by 1 issuccessfully transmitted.

On the contrary, if the non-AP STA specifies the TSF 0 offset value by 0in the U-APSD coexistence element, the AP does not transmit a frame tothe non-AP STA in the outside of the U-APSD existence service period.The U-APSD existence service period starts when the AP receives a U-APSDtrigger frame and ends after a transmission period specified by Formula11 in the following.

End of transmission period=T+Duration  [Formula 3]

In Formula 3, T indicates time of receiving the U-APSD trigger frame bythe AP. And, Duration indicates a firstly appearing value among aduration/interval field value of the U-APSD coexistence element andtiming on which an EOSP (end of service period) bit configured by 1 issuccessfully transmitted.

If the AP determines that the AP has a frame to transmit more during theU-APSD coexistence service period and the frame will be successfullytransmitted before the service period expires, an additional (more) bitcan be configured by 1.

If a frame is anticipated as a last frame to be transmitted to thenon-AP STA during the U-APSD coexistence service period, the AP canconfigure the EOSP bit by 1 in the corresponding frame. If the lastframe is not successfully transmitted to the non-AP STA before theU-APSD coexistence service period expires, the AP transmits a QoS nullframe of which the EOSP bit is configured by 1. The non-AP STA can entera doze state when the U-APSD coexistence service period expires.

TIM Element Structure

FIG. 20 is a diagram for an example of a TIM element format.

Referring to FIG. 20, a TIM element can include an element ID field, alength field, a DTIM count field, a DTIM period field, a bitmap controlfield and a partial virtual bitmap field. The length field indicateslength of an information field. The DTIM count field indicates how manybeacon frames (including a current frame) exist before a next DTIM istransmitted. The TIM period field indicates the number of beaconintervals between contiguous DTIMs. If all TIMs correspond to a DTIM,the DTIM period field has a value of 1. The DTIM period value isreserved by 0 and configured by 1 octet. The bitmap control fieldconsists of one octet. A bit 0 of the bitmap control field is a trafficindicator bit for AID 0. If one or more group addressed MSDUs (MACservice data unit)/MMPUDs (MAC management protocol data unit) have datato transmit in an AP or a mesh STA, the DTIM count field is set to 0 andthe bit 0 of the bitmap control field is set to 1. Remaining 7 bits ofthe first octet indicates a bitmap offset. A traffic indication virtualbitmap indicated by an AP generating a TIM or the mesh STA is consistedof 2008 bits (=251 octets). A bit number N (where 0≦N≦2007) of a bitmapcan be indicated by an octet number N/8 and a bit number (N mod 8). Eachbit of the traffic indication virtual bitmap indicates whether thereexist data to be transmitted by an AP. If there exist data to betransmitted by the AP for an individually addressed MSDU/MMPDU (AID=N),the bit number N is set to 1. If there does not exist data to betransmitted, the bit number N is set to 0.

The above-mentioned fields correspond to an example of fields capable ofbeing included in a TIM element. Each of the fields can be replaced witha different field or an additional field can be further included.

As an example of a method of compressing a bitmap of a TIM element, itmay consider a method of changing (reassigning) an AID of an STAaccording to a traffic pattern. Regarding this, it shall be describedwith reference to FIG. 21 in the following.

FIG. 21 is a diagram for an example of compression of a TIM elementusing dynamic AID assignment.

Referring to FIG. 21, if an AP has data to transmit to an STA, a bitindicating an AID of the STA in a bitmap of a TIM element is set to 1.If the AP does not have data to transmit to the STA, the bit indicatingthe AID of the STA in the bitmap of the TIM element is set to 0. On thecontrary, if the AP has data to transmit to the STA, the bit indicatingthe AID of the STA in the bitmap of the TIM element may be set to 0. Ifthe AP does not have data to transmit to the STA, the bit indicating theAID of the STA in the bitmap of the TIM element may be set to 1. FIG. 21shows an example that the AP has data to transmit to STAs to which AID2, 6 and 10 are assigned. In this case, if an AID of an STA to which theAID 6 is assigned is modified (reassigned) into 1 and an AID of an STAto which the AID 10 is assigned is modified (reassigned) into 3, a sizeof the bitmap configuring the TIM element can be reduced. Specifically,if the AP has data to transmit to the STAs to which original AID 2, 6and 10 are assigned, according to a legacy TIM element, bits, which arepositioned between the bits indicating the AIDs of the STAs in whichdata exist, should be included in the bitmap to indicate the STAs. Forinstance, the bits (bits indicating original AID 3 to 5) positionedbetween the bit indicating the original AID 2 and the bit indicating theoriginal AID 6 and the bits (bits indicating original AID 7 to 9)positioned between the bit indicating the original AID 6 and the bitindicating the original AID 10 should be included in the bitmap. Yet, ifthe AIDs of the STAs in which data to be transmitted by the AP exist arechanged (reassigned) in a manner of being contiguously configured, sincethe bitmap is configured by excluding bits positioned between bitsindicating modified AIDs, the size of the bitmap can be reduced.

When an AID assigned to an STA is dynamically changed according to atraffic patter and the like, in order to efficiently reduce a size of abitmap according to the change of the AID, it is preferable to assignAIDs to the STAs with a prescribed space between the AIDs rather thancontiguously assign the AIDs to the STAs. Yet, in case of assigning theAIDs to the STAs with a prescribed space between the AIDs, a size of awhole bitmap may increase. Hence, it may also be necessary to change abitmap encoding scheme to be suitable for the aforementioned content.

The present invention proposes a structure of a TIM element used for notonly effectively supporting the aforementioned dynamic AID assignmentbut also efficiently compressing a bitmap.

Enhanced TIM Structure

FIG. 22 is a diagram for explaining a format of a TIM element.

Referring to FIG. 22, an example of a bitmap form of a TIM element usedfor informing whether there exist downlink data buffered for STAsbelonging to a range ranging from AID n to AID m is shown.

A TIM element can be configured in a manner of including an offsetfield, a length field and a bitmap field. These fields can be includedin a partial virtual bitmap field of the TIM element mentioned earlierin FIG. 20. Each of the fields corresponds to an example of fieldscapable of being included in the TIM element. Each field can be replacedwith a different field and an additional field can be further included.

The offset field means a start of a bitmap. In particular, the offsetfield indicates a start point of an AID range where traffic is indicatedby a corresponding TIM element. In an example of FIG. 22, since the TIMelement corresponds to a TIM element for STAs belonging to a rangeranging from an AID n to an AID m, the offset field has a value of theAID n. The length field indicates a length of a bitmap. In particular,the length field indicates the AID range where traffic is indicated by acorresponding TIM element. In this case, a unit (e.g., octet) of thelength field can be represented by a configuration unit of a bitmap. InFIG. 22, the length field has values indicating a range (or numbers)ranging from the AID n to the AID m. The bitmap field indicates whetheran AP stores downlink data buffered for STAs belonging to a rangeranging from an AID indicated by the offset field value to an AIDindicated by the length field by 0 or 1. In FIG. 22, the bitmap fieldindicates whether an AP stores downlink data buffered for the STAsbelonging to the range ranging from the AID n to the AID m by 0 or 1.

In this case, a method of configuring a bitmap can be mainly classifiedinto two. First of all, when a bitmap is configured in a manner thateach of bits included in the bitmap respectively indicates an AIDcorresponding to the each bit, it is able to configure the bitmap forsequentially increasing AIDs as much as 1. This sort of method may benamed a sequential bitmap. And, when a bitmap is configured in a mannerthat each of bits included in the bitmap respectively indicates an AIDcorresponding to the each bit, it is able to configure the bitmap forsequentially increasing AIDs as much as a prescribed value (hereinafter,this is called delta). This sort of method may be named a linear bitmap.Regarding this, it shall be described with reference to FIG. 23 in thefollowing.

FIG. 23 is a diagram for explaining a bitmap format of a TIM elementaccording to one embodiment of the present invention.

FIG. 23 (a) shows an example of a sequential bitmap and FIG. 23 (b)shows an example of a linear bitmap. Similar to FIG. 22, FIG. 23 showsan example of a bitmap for STAs belonging to a range ranging from an AIDn to an AID m.

In case of a sequential bitmap, a first position of the bitmap indicatesa traffic indication for an STA (in FIG. 23, an STA to which an AID n isassigned) including an AID value indicated by an offset field. Thetraffic indication can be indicated for STAs including a sequentiallyincreasing AID value as much as 1 from the first bit position of thebitmap via contiguous bitmap positions. FIG. 23 shows that an AP storesframes (downlink data) buffered for STAs to which an AID n and an AIDn+8 are assigned, respectively.

Similar to the sequential bitmap, in case of a linear bitmap, a firstposition of the bitmap indicates a traffic indication for an STA (inFIG. 23, an STA to which an AID n is assigned) including an AID valueindicated by an offset field. Yet, the traffic indication can beindicated for STAs including increasing AID value as much as delta fromthe first bit position of the bitmap via contiguous bitmap positions. InFIG. 23, a delta value corresponds to 8. FIG. 23 shows that an AP storesframes (downlink data) buffered for STAs to which an AID n and an AIDn+8 are assigned, respectively. In this case, the delta value maycorrespond to a value equal to or less than a bitmap configuration unit(e.g., a divisor of the bitmap configuration unit). In case of applyinga linear bitmap encoding scheme, an AP may inform an STA of a deltavalue in an association process via system information. Or, the AP mayinform the STA of the delta value via a corresponding TIM element.

When a configuration unit of a bitmap corresponds to 1 octet (8 bits),if a sequential bitmap is used, 2 octets are required for a bitmapencoding. Yet, if a linear bitmap is used, 1 octet is sufficient for thebitmap encoding. This is because, since the AP has no traffic bufferedfor STAs including an AID n+1, an AID n+9, an AID n+17, an AID n+25, anAID n+33, an AID n+41, an AID n+49 and an AID n+57, corresponding partcan be excluded when a bitmap is configured. In this case, as mentionedin the foregoing description, if a configuration unit of a bitmapcorresponds to 1 octet, a unit of a length field may correspond to octetas well. Hence, a length field value of a sequential bitmap becomes 2and a length field value of a linear bitmap becomes 1.

Meanwhile, a TIM element according to a legacy definition is notsuitable for application of an M2M application where the great number(e.g., exceeding 2007) of STAs are capable of being associated with oneAP. In case of extending a structure of the legacy TIM element as it is,it is not able to support the structure with a legacy frame format sincea bitmap size of the TIM element is extended too much. And, the TIMelement according to the legacy definition is not appropriate for M2Mcommunication considering an application of a low transmission rate.And, it is expected that there is few STA including a reception dataframe in one beacon interval in the M2M communication. Hence,considering the above-mentioned application example of the M2Mcommunication, it is expected that most of bits will have value of 0although a size of a bitmap of a TIM element is extended. Hence, it isnecessary to have a technology of efficiently compressing a bitmap.

To this end, it is able to make a TIM element have a hierarchicalstructure. Regarding this, it shall be described with reference to FIG.24 in the following.

FIG. 24 is a diagram for explaining a hierarchical structure of a TIMelement.

FIG. 24 shows an example of a hierarchical structure of a TIM elementincluding a layer of 3 levels. In the structure of the layer of 3levels, a total AID space capable of maximally supporting STAs isdivided into a plurality of page groups and each of a plurality of thepage groups is divided into a plurality of blocks. And, each of aplurality of the blocks can be divided into a plurality of sub-blocks.Although the example shown in FIG. 24 shows the layer of 3 levels, a TIMelement of a hierarchical structure can be configured by a form of 2levels or a form of more than 3 levels. In the example of FIG. 24, thetotal AID space is divided into total 4 page groups, one page group isdivided into 32 blocks and one block can be divided into 8 sub-blocks.If one sub-block has a size of 1 octet, one sub-block can support 8STAs, one block can support 64 (8*8) STAs and one page group can supporttotal 2048 (64*32) STAs. Yet, the example of FIG. 24 is just an example.The number of page groups by which the total AID space is divided, thenumber of blocks by which one page group is divided and the number ofsub-blocks by which one block is divided can be differently configured.

As mentioned in the foregoing description, AID(s) belonging to aspecific page group can be included in one TIM element by dividing thetotal AID space into a plurality of page groups. A channel access ofSTA(s) corresponding to AID(s) belonging to a specific page group ispermitted only for a specific time interval (e.g., a beacon intervalincluding a corresponding TIM element) and a channel access of remainingSTA(s) can be restricted for the specific time interval. In particular,a prescribed time interval permitted to a specific STA(s) only can benamed a RAW (restricted access window). By permitting a channel accessto STA(s) corresponding to a specific page group only for a specifictime interval, a channel access can be permitted according to a pagegroup on a different time interval. By doing so, a TIM element shortageproblem for many numbers of STAs can be solved and efficient datatransmission/reception can be performed.

As mentioned in the foregoing description, if a TIM element isconfigured by a hierarchical structure, an AID structure can bedetermined based on the TIM element including the hierarchicalstructure. Regarding this, it shall be described with reference to FIG.25 in the following.

FIG. 25 is a diagram for an example of an AID structure according to astructure of a hierarchical TIM element.

Referring to FIG. 25, it shows an example of an AID based on a TIMelement, which is configured under an assumption that the TIM element isconfigured by a layer of 3 levels. The AID can be configured by a pagegroup identifier, a block index, a sub-block index and bits used forindicating a bit position index of a corresponding STA in a sub-blockaccording to a hierarchical structure of the TIM element. In particular,several bits of the front of the AID, next several bits, next severalbits and next several bits can indicate a page group, a block index, asub-block index and a bit position index of a corresponding STA in asub-block, respectively. In an example shown in FIG. 25, first 2 bits ofthe AID indicates a page group identifier among the total 4 page groups,next 5 bits indicates a block index among the total 32 blocks, next 3bits indicates a sub-block index among the total 8 sub-blocks and next 3bits indicates a bit position index of a corresponding STA in asub-block. Hence, an AID can be assigned to an STA in a manner of beinggroup by a page group, a block and a sub-block. By doing so, the STA cancheck a bit position indicating the STA in a bitmap of the TIM elementvia the AID assigned to the STA.

As mentioned in the foregoing description, when a TIM element isconfigured by a hierarchical structure, a bitmap of the TIM element canbe configured in a manner of being divided into sub-bitmaps (sub-blockbitmaps). Regarding a format of the TIM element, it shall be describedwith reference to FIG. 26 in the following.

FIG. 26 is a diagram for an example of a format of a TIM elementincluding a hierarchical structure.

FIG. 26 shows an example of a bitmap form of a TIM element indicatingwhether there exist downlink data buffered for STAs belonging to a rangeranging from an AID n to an AID m.

A TIM element can be configured in a manner of including an offsetfield, a bitmap control field and a bitmap field. The offset field meansa start of a bitmap. In particular, the offset field means a start pointof an AID range where traffic is indicated by the TIM element. In anexample of FIG. 22, since the TIM element indicates TIM element for STAsbelonging to a range ranging from an AID n to an AID m, the offset fieldhas a value of the AID n. The bitmap control field is used for thepurpose of indicating sub-bitmap to show which sub-bitmaps are used toconfigure the bitmap field. In particular, the bitmap field can beconfigured by the sub-bitmaps indicated by the bitmap control field. Forinstance, as shown in FIG. 26, when it is assumed that there is 8sub-bitmaps capable of being included in the bitmap field, if the bitmapcontrol field includes 1, 0, 1, 0, 0, 0, 0, 0, it means that the bitmapfield is configured by a first sub-bitmap and a third sub-bitmap only.

Each of the fields corresponds to an example of fields capable of beingincluded in the TIM element. Each field can be replaced with a differentfield and an additional field can be further included. For instance, inthe example shown in FIG. 20, if the partial virtual bitmap field of theTIM element is encoded by a block level, the partial virtual bitmapfield can include one or more blocks included in one page group and eachof the aforementioned fields can be included in one block. In this case,the offset field and the bitmap control field can be replaced with ablock offset field and a block control field or a block bitmap field,respectively. Regarding this, it shall be described with reference toFIG. 27 in the following.

FIG. 27 is a diagram for an example of a format of a TIM elementincluding a hierarchical structure.

Referring to FIG. 27, a plurality of AIDs are indicated by one sub-blockand a plurality of sub-blocks are indicated by one block. In an exampleof FIG. 27, one sub-block covers 8 AIDs and one block covers 8sub-blocks.

A TIM element can be configured in a manner of including a block offsetfield, a block control field, a block bitmap field and a bitmap field(or sub-block field). The block offset field is used for the purpose ofindicating a position of a block in a bitmap of the total TIM element.In case that there exist various methods of indicating the TIM element(or bitmap encoding schemes), the block control field is used for thepurpose of indicating the method of indicating the TIM element. If thereexists a sub-block (or sub-block bitmap) to which an AID paged bycontiguous bit positions belongs thereto from a first bit position, theblock bitmap field indicates the sub-block. In particular, an n^(th) bitof the block bitmap field indicates whether there exists a bitmap of ann^(th) sub-block in the bitmap field. In an example of FIG. 27, a first,a third and a seventh sub-block correspond to the sub-block. The blockbitmap field has 1, 0, 1, 0, 0, 0, 1, 0 to indicate the sub-blocks. Anm-th bit position of a sub-block bitmap indicates whether an m-th STAhas data buffered for an AP.

In FIG. 27, since a 8-byte traffic indication bitmap block can becompressed to a 5-byte encoded bitmap using a block bitmap encodingscheme, overhead of a TIM can be reduced.

In general, in order to transmit a TIM in a manner of being included ina beacon frame and in order for most STAs within a BSS to receive thebeacon frame including the TIM, a low MCS (modulation and coding scheme)should be applied. In this case, in terms of efficient utilization of aresource, it would be mandatory to reduce a size of the TIM.

As mentioned in the foregoing example, in case of using the block bitmapencoding scheme, overhead of a TIM can be reduced. Yet, when the numberof sub-block bitmaps indicated by one block is small, overheadconfigured to indicate a sub-block bitmap occupies a considerable partcompared to an actually delivered sub-block bitmap.

For instance, if there exists one sub-block bitmap per block, overhead(block control of 3 bits, block offset of 5 bits and block bitmap of 1byte) of 2 bytes are added to a sub-block bitmap of 1 byte. Inparticular, when information of 1 byte is transmitted, 2 bytes areadditionally added. In particular, overhead of 200% occurs. If thereexist two sub-block bitmaps per block, since 2 bytes are additionallyadded (block control of 3 bits, block offset of 5 bits and block bitmapof 1 byte) to sub-block bitmaps of 2 bytes, overhead of 100% may occur.If there exist three sub-block bitmaps per block, since 2 bytes areadditionally added (block control of 3 bits, block offset of 5 bits andblock bitmap of 1 byte) to sub-block bitmaps of 3 bytes, overhead of 50%may occur.

FIG. 28 is a diagram for explaining an example of overhead occurringwhen one sub-block bitmap per block is indicated in three contiguousblocks. In FIG. 28, assume that one sub-block per each block isindicated. As shown in an example of FIG. 28, since one block controlfield, one block offset field and one block bitmap field are used forone block, overhead of minimum 6 bytes ((3 bits (block control field)+5bits (block offset field)+1 byte (block bitmap field))*3) should beadded to compress 3 blocks. Since one sub-block per block is indicated,overhead of 6 bytes should be added to indicate a sub-block of 3 bytes.

As mentioned in the foregoing description, if the number of bitmapsindicated in a random block is small, since overhead is greater thandata to be transmitted, a problem that transmission rate becomes worseoccurs. In order to solve the problem, the present invention proposes anenhanced encoding scheme in the following.

Enhanced Encoding Scheme

The present invention proposes an enhanced encoding scheme capable ofindicating N (N is a natural number equal to or greater than 2) numberof blocks. In this case, the N may correspond to a value fixed in asystem or a value randomly selected by an STA. If the N is fixedlymanaged by a system, a long block bitmap encoding scheme can be applied.If the N is managed in a manner of being randomly selected by an STA, amultiple block bitmap encoding scheme can be applied. Each of theencoding schemes is explained in detail in the following.

Embodiment 1 Long Block Bitmap Encoding Scheme

A TIM element to which a long block bitmap encoding scheme is appliedcan be used for indicating N number of blocks fixed by a system. To thisend, a block control field can indicate that there exist maximum Nnumbers of contiguous block bitmaps. The block bitmap field may have asize of N*8 bits. As an example, if the N corresponds to 2, the blockbitmap field may have a size of 16 bits. If the N corresponds to 3, theblock bitmap field may have a size of 24 bits.

Consequently, the TIM element to which the long block bitmap encodingscheme is applied can include a block control field of 3 bits, a blockoffset field of 5 bits, a block bitmap field of N*8 bits and a sub-blockbitmap field of a size, which is variable according to the number ofindicated sub-blocks.

FIGS. 29 to 31 are diagrams for explaining examples of a long blockbitmap encoding scheme.

FIG. 29 shows a TIM element in case that N corresponds to 2 and FIG. 30shows a TIM element in case that N corresponds to 3. For clarity ofexplanation, assume that there is one sub-block including an AID, whichis paged in each block.

As shown in an example of FIG. 29, a TIM element, which is used forindicating 2 blocks and applied a long block bitmap encoding scheme, caninclude a block control field of 3 bits, a block offset field of 5 bitsand a block bitmap field of 2 bytes. In FIG. 29, since one block has anAID in which one sub-block is paged, one sub-block field has a size oftotal 2 bytes.

The block control field indicates that the TIM element is encoded by thelong block encoding scheme. An STA checks an encoding scheme indicatedby the block control field and may be then able to recognize that theTIM element indicates the N number of blocks.

The block bitmap field indicates a sub-block (or sub-block bitmap) towhich any AID paged in N number of blocks belongs thereto. The blockbitmap field can indicate one block in 8-bit unit. As an example, asshown in FIG. 29, in order to indicate two blocks, the block bitmapfield should have a length of minimum 16 bits. And, as shown in FIG. 30,in order to indicate three blocks, the block bitmap field should have alength of minimum 24 bits.

Hence, in FIG. 29, it may be comprehended as the first 8 bits (#0) areused for indicating sub-blocks of a first block (block #0) and remaining8 bits are used for indicating sub-blocks of a second block (block #1).

In an example shown in FIG. 29, since a sub-block 2 of the first blockand a sub-block 5 of the second block have a paged AID, the block bitmapfield may have a value of 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0,0.

In case of using a legacy encoding scheme, 2 blocks can be indicatedusing 2 TIM elements including a block control field of 3 bits, a blockoffset field of 5 bits and a block bitmap field of 1 byte. As shown inthe example of FIG. 29, in case of using the long block bitmap encodingscheme, since 3 bytes are additionally required only except thesub-block bitmap field (3 bits (block control field)+5 bits (blockoffset field)+2 bytes (block bitmap field)), it is able to obtain a gainof 1 byte compared to the legacy encoding scheme requiring additional 4bytes ({3 bits (block control field)+5 bits (block offset field)+1 byte(block bitmap field)}*2).

As shown in an example of FIG. 30, a TIM element, which is used forindicating 3 blocks and applied a long block bitmap encoding scheme, caninclude a block control field of 3 bits, a block offset field of 5 bitsand a block bitmap field of 3 bytes. In FIG. 30, since one block has anAID in which one sub-block is paged, one sub-block field has a size oftotal 3 bytes.

In this case, the first 8 bits (#0) of the block bitmap field are usedfor indicating sub-blocks of a first block (block #0), next 8 bits (#1)are used for indicating sub-blocks of a second block (block #1) andremaining 8 bits can be used for indicating sub-blocks of a third block(block #2).

In an example shown in FIG. 30, since a sub-block 2 of the first block,a sub-block 5 of the second block and a sub-block 7 of the third blockhave a paged AID, the block bitmap field may have a value of 0, 1, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0.

In case of using a legacy encoding scheme, 3 blocks can be indicatedusing 3 TIM elements including a block control field of 3 bits, a blockoffset field of 5 bits and a block bitmap field of 1 byte. As shown inthe example of FIG. 30, in case of using the long block bitmap encodingscheme, since 4 bytes are additionally required only except thesub-block bitmap field (3 bits (block control field)+5 bits (blockoffset field)+3 bytes (block bitmap field)), it is able to obtain a gainof 2 bytes compared to the legacy encoding scheme requiring additional 6bytes ({3 bits (block control field)+5 bits (block offset field)+1 byte(block bitmap field)}*2).

In case of using the legacy encoding scheme to indicate the N number ofblocks, the TIM element requires a size of N*2 bytes except thesub-block bitmap field. On the contrary, in case of using the long blockbitmap encoding scheme, the TIM element requires a size of 1 byte (blockcontrol field of 3 bits and block offset field of 5 bits)+N*1 byte(block bitmap field) except the sub-block bitmap field. Hence, the longblock bitmap encoding scheme can obtain a gain of N−1 byte compared tothe legacy encoding scheme.

FIG. 29 and FIG. 30 shows examples that the TIM element includes oneblock bitmap field of N bytes. On the contrary, a block bitmap sizefield of 1 byte can be repeatedly positioned one by one. For instance,as shown in an example of FIG. 31, if a size of the block bitmap fieldindicated by the block bitmap size field corresponds to 3 bytes, theblock bitmap field and the sub-block bitmap filed can be repeatedlypositioned by four times. In the example shown in FIG. 31, a first blockbitmap field (#0) is used for indicating a first block (#0) and a secondblock bitmap field (#1) can be used for indicating a second block (#1).In particular, an N-th block bitmap field can be used for indicating anN-th block.

Although FIG. 31 shows an example that a size of all sub-block bitmapfields positioned between the block bitmap fields corresponds to 1 byte,the size of the sub-block bitmap fields may vary according to the numberof sub-blocks including a paged AID in a block.

Embodiment 2 Multiple Block Bitmap Encoding Scheme

A TIM element to which a multiple block bitmap encoding scheme isapplied can be used for indicating N number of blocks which aresubjectively determined by an AP. To this end, a block bitmap size fieldcan be further included in the TIM element to which the multiple blockbitmap encoding scheme is applied compared to the TIM element to whichthe long block bitmap encoding scheme is applied.

The block bitmap size field indicates a size of a block bitmap field.Specifically, the block bitmap size field can indicate the size of theblock bitmap field in a byte unit or a block unit. Specifically, in casethat the TIM element indicates N number of blocks, the block bitmap sizefield can indicate that the size of the block bitmap field correspondsto N bytes. For instance, if the TIM element is configured to indicate 4blocks, the block bitmap size field can indicate that the block bitmapfield corresponds to 4 bytes.

FIG. 32 and FIG. 33 are diagrams for explaining examples of a multipleblock bitmap encoding scheme. For clarity of explanation, assume thatthe TIM element indicates 4 blocks and there is one sub-block includingan AID paged in each block.

As shown in FIG. 32, the TIM element, which is used for indicating 4blocks and applied a multiple bitmap encoding scheme, can include ablock control field of 3 bits, a block offset field of 5 bits, a blockbitmap size field of 1 byte and a block bitmap field of 2 bytes. In FIG.32, since the TIM element is used for indicating the 4 blocks, the blockbitmap size field can indicate a size of a block bitmap with 1 byte.

Having received the TIM element, an STA decodes the block bitmap sizefield and may be then able to recognize the size of the block bitmapfield and the number of blocks indicated by the TIM element.

As shown in the example of FIG. 32, a block bitmap field of a sizeindicated by the block bitmap size field can be positioned immediatelyafter the block bitmap size field. As mentioned earlier in FIG. 29 toFIG. 31, the block bitmap field of a size of 4 bytes can indicate ablock in 8-bit unit.

The block bitmap size field of 1 byte can be repeatedly positioned oneby one. For instance, as shown in an example of FIG. 33, if a size ofthe block bitmap field indicated by the block bitmap size fieldcorresponds to 4 bytes, the block bitmap field and the sub-block bitmapfiled can be repeatedly positioned by four times. In the example shownin FIG. 33, a first block bitmap field (#0) is used for indicating afirst block (#0) and a second block bitmap field (#1) can be used forindicating a second block (#1). In particular, an N-th block bitmapfield can be used for indicating an N-th block.

Although FIG. 33 shows an example that a size of all sub-block bitmapfields positioned between the block bitmap fields corresponds to 1 byte,the size of the sub-block bitmap fields may vary according to the numberof sub-blocks including a paged AID in a block.

In case of using the legacy encoding scheme to indicate the N number ofblocks, the TIM element requires a size of N*2 bytes except thesub-block bitmap field. On the contrary, in case of using the multipleblock bitmap encoding scheme, the TIM element requires a size of 1 byte(block control field of 3 bits and block offset field of 5 bits)+1 byte(block bitmap size field)+N*1 byte (block bitmap field) except thesub-block bitmap field. Hence, the long block bitmap encoding scheme canobtain a gain of N−2 byte compared to the legacy encoding scheme.

It is not necessary to independently implement the aforementionedembodiment 1 and the embodiment 2. The embodiment 1 and the embodiment 2can be used in a manner of being combined with each other in a wirelesscommunication system. For instance, an AP usually uses the long blockbitmap encoding scheme configured to indicate the N number of blocksdetermined by a system. If the AP intends to indicate blocks more thanthe N number of blocks determined by the system by one TIM element, theAP can apply the multiple block bitmap encoding scheme at last.

FIG. 34 is a block diagram of an example of a wireless device accordingto one embodiment of the present invention.

Referring to FIG. 34, an AP 10 includes a processor 11, a memory 12 anda transceiver 13. An STA (20) includes a processor 21, a memory 22 and atransceiver 23. The transceiver 13/23 can transmit/receive a radiosignal and can implement a physical layer according to IEEE 802 system.The processor 11/21 is connected with the transceiver 13/23 and canimplement the physical layer according to IEEE 802 system and/or MAClayer. The processor 11/21 can be configured to perform operationsaccording to the aforementioned various embodiments of the presentinvention. A module configured to implement operations of the AP andoperations of the STA according to the aforementioned variousembodiments of the present invention is stored in the memory 12/22 andcan be implemented by the processor 11/21. The memory 12/22 is includedin the inside of the processor 11/21 or installed in the outside of theprocessor 11/21 and can be connected with the processor 11/21 by awell-known means.

Specific configuration of the AP and the STA device can be implementedto independently apply the items explained in the aforementioned variousembodiments of the present invention or apply two or more embodiment atthe same time. For clarity, explanation on duplicated contents isomitted.

Embodiments of the present invention can be implemented using variousmeans. For instance, embodiments of the present invention can beimplemented using hardware, firmware, software and/or any combinationsthereof.

In the implementation by hardware, a method according to each embodimentof the present invention can be implemented by at least one selectedfrom the group consisting of ASICs (application specific integratedcircuits), DSPs (digital signal processors), DSPDs (digital signalprocessing devices), PLDs (programmable logic devices), FPGAs (fieldprogrammable gate arrays), processor, controller, microcontroller,microprocessor and the like.

In case of the implementation by firmware or software, a methodaccording to each embodiment of the present invention can be implementedby modules, procedures, and/or functions for performing theabove-explained functions or operations. Software code is stored in amemory unit and is then drivable by a processor. The memory unit isprovided within or outside the processor to exchange data with theprocessor through the various means known in public.

While the present invention has been described and illustrated hereinwith reference to the preferred embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention that come within thescope of the appended claims and their equivalents.

INDUSTRIAL APPLICABILITY

Although various embodiments according to the present invention areexplained centering on an example applied to IEEE 802.11 system, thepresent invention can be identically applied to various wireless accesssystems as well as IEEE 802.11 system.

What is claimed is:
 1. A method of transmitting a traffic indication map(TIM) to a station (STA) in a wireless communication system, comprising:transmitting the TIM to the STA via a beacon frame, wherein the TIMcomprises a block bitmap field, which indicates a sub-block containingan STA in which downlink data buffered for each of N (N is a naturalnumber equal to or greater than 2) number of blocks, and a block controlfield indicating an encoding scheme of the bitmap field.
 2. The methodof claim 1, wherein the block bitmap field has a size of N byte.
 3. Themethod of claim 2, wherein the block bitmap field of 1 byte unitindicates blocks different from each other.
 4. The method of claim 1,wherein the N corresponds to a fixed value in the wireless communicationsystem.
 5. The method of claim 1, wherein the N corresponds to a valueconfigured by an access point (AP).
 6. The method of claim 1, whereinthe TIM further comprises a block bitmap size field indicating a size ofthe block bitmap field.
 7. The method of claim 6, wherein the blockbitmap size field indicates the size of the block bitmap field by Nbyte.
 8. The method of claim 1, wherein the block bitmap field and asub-block bitmap field indicated by the block bitmap field arerepeatedly appeared by N times in the TIM.
 9. A method of receiving atraffic indication map (TIM), which is received by a station (STA) in awireless communication system, comprising: receiving the TIM from anaccess point (AP) via a beacon frame, wherein the TIM comprises a blockbitmap field, which indicates a sub-block containing an STA in whichdownlink data buffered for each of N (N is a natural number equal to orgreater than 2) number of blocks, and a block control field indicatingan encoding scheme of the bitmap field.
 10. The method of claim 7,wherein the block bitmap field has a size of N byte.
 11. The method ofclaim 7, wherein the N is a value configured by the access point (AP).12. The method of claim 7, wherein the TIM further comprises a blockbitmap size field indicating a size of the block bitmap field.
 13. Themethod of claim 12, wherein the block bitmap size field indicates thesize of the block bitmap field by N byte.
 14. A device transmitting atraffic indication map (TIM) to a station (STA) in a wirelesscommunication system, comprising: a transceiver configured to transmitand receive a radio signal; and a processor, the processor configured totransmit the TIM to the STA via a beacon frame, wherein the TIMcomprises a block bitmap field, which indicates a sub-block containingan STA in which downlink data buffered for each of N (N is a naturalnumber equal to or greater than 2) number of blocks, and a block controlfield indicating an encoding scheme of the bitmap field.
 15. A station(STA) device receiving a traffic indication map (TIM) in a wirelesscommunication system, comprising: a transceiver configured to transmitand receive a radio signal; and a processor, the processor configured toreceive the TIM from an access point (AP) via a beacon frame, whereinthe TIM comprises a block bitmap field, which indicates a sub-blockcontaining an STA in which downlink data buffered for each of N (N is anatural number equal to or greater than 2) number of blocks, and a blockcontrol field indicating an encoding scheme of the bitmap field.